1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method, and particularly relates to reduction of density unevenness, such as so-called joint stripe, which arises from a phenomenon in which an ink ejecting direction is deviated at a boundary between scanning areas of a print head.
2. Description of the Related Art
Heretofore, various image forming apparatuses for printing an image on a printing medium such as a paper sheet and an OHP sheet have been proposed as types in each of which a print head in any of various printing systems is provided. The print heads are available in a wire dot system, a thermal system, a thermal transfer system, an inkjet system, or the like. Especially, the inkjet system has been attracting attention as a system that is low in running cost and is capable of carrying out a highly silent printing operation because the system directly send inks to a printing paper sheet
Furthermore, a common type of the above-described printing apparatus is a so-called serial type printing apparatus in which a carriage mounted with a print head moves so that the print head scans a printing medium and performs printing thereto. This serial-type printing apparatus conveys a printing medium by a predetermined amount in a direction orthogonal to the scanning direction after every scan by the print head, and prints an image onto the printing medium according to print data by alternately repeating the scan by the print head and the conveying of the printing medium. This serial printing system includes: a one-pass printing method in which printing an image in a unit area is completed in one scan; and a multi-pass printing system in which printing an image in a unit area is completed in multiple scans.
At a boundary between unit areas in each of which a printed image is completed in a single scan or multiple scans, ink density unevenness in the form of a white stripe may be caused, especially when a density of ink ejected from a print head (print duty) is high.
This phenomenon is considered to be caused when ink ejected from ejection openings located near an end portion of a print head is drawn toward the center side of the print head due to a factor, such as air current. To be more specific, ink from the ejection openings located near an end portion of an ejection opening array of a print head is ejected in deviated directions, and lands on a location moved to the center side of an area for which a printed image is completed. As a result, no ink dot is formed near the boundary between the unit areas, and a white stripe is generated. Since ink ejected from an ejection opening located near an end portion of an ejection opening array is drawn towards the center side of the print head as described above, hereinafter, this phenomenon is called end deviation. When a white stripe is generated due to the end deviation, printing quality is significantly lowered.
A method to prevent the occurrence of the white stripe due to the end deviation and to obtain a high-quality image has been proposed. Japanese Patent Laid-open No. 2004-168003 describes a configuration in a serial scan system in which scanning areas on the printing medium are arranged to partly overlap each other by setting a conveying amount of a printing medium to be smaller than a normal amount so that the white stripes due to the end deviation can be made less noticeable in a high-density region of an image which has a high printing duty. In this technique, on the other hand, in a region in which no end deviation occurs because of a low printing duty, a black stripe may occur due to the overlap of the scanning areas. Thus, it is also proposed that printing data for ejection openings near the end portion is thinned out by an appropriately amount.
However, investigation by the inventor of the present application has shown that the end deviation is not a phenomenon confined to and occurring in each one of ejection opening arrays which are separately provided for respective ink colors or the like. To be more specific, an amount of the end deviation occurring at one ejection opening array is not determined according to only a printing duty of ejection openings near the end portion of the ejection opening array, but is also affected by a printing duty of other adjacent ejection opening arrays. Accordingly, thinning processing performed to prevent the occurrence of the black stripe caused by an overlap between image completing areas, in the case that a printing duty is low and therefore the end deviation does not occur, may not be appropriate.
FIG. 1 is a view schematically illustrating the effect of other adjacent ejection opening array. As illustrated in FIG. 1, a print head 5 is provided with an ejection opening array (nozzle array) for cyan (C) and an ejection opening array (nozzle array) for magenta (M) arranged in a scanning direction in this order. Then, when scan for printing is performed, especially in the case where a printing duty of the C nozzle array located anteriorly in the scanning direction is high, a flow field indicated by arrows occurs near the M nozzle array. In this case, since large air currents flow into the vicinity of the M nozzle array, drawing (end deviation) of ejected ink due to a reduced pressure between nozzles in the M nozzle array is reduced, and thus the size of white stripe is different from that caused by a single nozzle array.
Furthermore, the occurrence of white stripe regarding a certain nozzle array is affected mostly by air currents from an adjacent ejection opening array located anteriorly thereto in a scanning direction, and also affected not only by the anterior ejection opening array, but also by air currents from a posterior ejection opening array. Especially, in the case where a scanning speed of a print head is slow or where a distance between nozzle arrays is small, the effect of such air currents is more significant.
As described above, if the thinning processing on printing data for a joining portion (overlapping region) is preformed according to a printing duty of only one of the ejection opening arrays, as described in Japanese Patent Laid-Open No. 2004-168003, also in the case where multiple ejection opening arrays are arranged next to each other, the resultant thinned-out printing data is unsuitable for an amount of end deviation which actually occurs. As a result, it is impossible to perform high-quality printing dealing with the end deviation with high accuracy.